Some people have noticed that the dates on the USCIS site are not necessarily a sign that your application isn't being processed. They will not move the date ahead until they've completed all applications from that date. So, if in July there was a tricky application that required more time to make it through the process, it can hold up the dates on the web site, but won't hold up processing. Once they are done with all of the July x files, they will update the date on the site. It is not unheard of to see the site list July 3rd one day, and Sept 1 the next. While I can not guarantee this is the case for you, I hope that it gives you some peace of mind.
I would be very surprised if your application is not completed in time for you to move next December. That's more than a year away, and if your application is straight forward, I wouldn't think there would be a long delay.
I will be sending you lots of good vibes! I know the visa process is hard, but it will be well worth it!!